New argon2i password encoder

[chalasr]

See symfony/symfony#21604 (3.4)
Can be documented around https://symfony.com/doc/current/reference/configuration/security.html#using-the-pbkdf2-encoder-security-and-speed

[chalasr]

The fact it won the password hashing competition and is the recommended algorithm for hashing passwords should probably be mentioned (per https://password-hashing.net).

[javiereguiluz]

See also https://symfony.com/blog/new-in-symfony-3-4-argon2i-password-hasher

It should also be mentioned that Bcrypt is a perfectly valid choice and that there's no need to update your existing applications to rehash passwords.

[xabbuh]

see #8481

[javiereguiluz]

Fixed by #8481.